Process for the deposition of gold or gold alloys



United States Patent O PROCESS FOR THE DEPOSITION OF GOLD OR GOLD ALLOYS Victor Spreter and Jean Mermillod, Geneva, Switzerland No Drawing. Application July 21, 1952, Serial No. 300,126

Claims priority, application Switzerland April 28, 1952 7 Claims. (Cl. 204-44) In most electroplating processes for the deposition of a metal or of an alloy of metals, making use of aqueous solutions of salts of the metal or metals from being deposited, and particularly regarding the deposition of gold or gold alloys, it is found that the metal deposit at the cathode is smaller in weight than the weight which should have been theoretically deposited for the consumed current. In other words, the yield of the operation, considered from the current consumption point of view, is not 100% but less, and possibly much less than the theoretical yield. A part of the current produces secondary electrolysis, and, in particular, releases hydrogen against the cathode. All metals, at the time of their electrolytic deposition, have the property of retaining the nascent hydrogen produced at the cathode.

This retained hydrogen in the metal is the cause of the porosity of thin deposits and of the extreme fragility of the thick ones, the included hydrogen creating internal stresses in the metal.

The noble metals are the most capable of retaining considerable quantifies of hydrogen. One never finds, during electrolysis of solutions of gold or gold alloys salts, the slightest evolution of hydrogen at the cathode, even when only 20% of the consumed current serves for metal precipitation; this is proof that the freed hydrogen is really retained in full by the gold deposit.

The present invention comprises a process for the deposition of gold or gold alloys by electroplating, and a bath for carrying out said process, which permit of avoiding the drawbacks due to the production of hydrogen at the cathode, by preventing said hydrogen to be retained by the coating formed.

This process consists in passing, as in already known processes, an electric current through an aqueous bath containing in solution an alkaline aurocyanide, the object' to be plated being at the cathode. It is characterized by the use of a bath containing, in the dissolved state, a nitrated derivative of an aliphatic organic compound of low molecular weight.

In fact it has been found that such derivatives, and in particular nitro-guanidine and nitro-urea, are capable of chemically reacting with the hydrogen formed at the cathode and thus of preventing that this gas should be retained by the gold deposit.

These derivatives fulfill the following conditions which must be observed in the processes for electrolytically depositing gold or gold alloys by means of alkaline aurocyanides baths, when good results are sought:

To have a very great afiinity for cathodic hydrogen;

To be able to act at comparatively low concentrations while consuming all the hydrogen produced;

To be soluble in water, even in the presence of the salts contained in the electrogilding baths and not to react with said salts;

To be comparatively stable at the working temperatures of electrogilding baths (20 to 80 C.);

To give reduction or decomposition products (in case of excessive bath temperatures) having no influence on the electrolysis;

Not to affect the brilliance of the deposits.

The bath according to the invention consists of an aqueous solution containing an alkaline aurocyanide; it is characterized by the fact that it contains, in a dissolved state, a nitrated derivative of an aliphatic organic compound of low molecular weight, such, for instance, as nitro-guanidine and nitro-urea, which are particularly 2,702,271 Patented Feb. 15, '1955 suitable. Either may be used at a concentration of only 5 to 10 grams per litre of bath. It is then found that all the hydrogen produced at the cathode reacts with one or the other of these compounds. Indeed, the thick gold or gold alloy deposits thus obtained are flexible and ductile, free from internal stresses; they do not scale off or break, even when their thickness reaches several tens of microns. On the other hand, these deposits, if quickly heated to a temperature exceeding 400 C., do not deteriorate and blister.

The following are two examples of carrying out the invention process for depositing brilliant coatings of a gold and copper alloy assaying approximately 20 carats.

Example 1 One prepares a gilding bath having the following composition:

G. per litre Gold as gold and potassium cyanide 2 Cu as copper and potassium cyanide 20 Potassium cyanide 10 Potassium ortho-phosphate (monohydrogen) 25 Potassium carb n 10 Nitro-guanidine 8 One prepares a gilding bath having the following composition:

G. per litre Gold as gold and potassium cyanide 2 Cu as copper and potassium cyanide 20 Potassium cyanide 10 Potassium ortho-phosphate (monohydrogen) 25 Potassium carbonate 10 Nitrourea 10 One places in this bath an unattackable anode and at the cathode the object to be coated with a brilliant gold alloy deposit. The bath is kept at a temperature between 25 and 35 C. An electric current is passed through it, of a density of 0.3 to 0.5 amp. per sq. decimeter (2.8 to 4.65 amp. per sq. foot) of the surface of the object to be plated. After one hour there results a deposit of 5 to 6 microns thickness, of a gold alloy of an approximate 20 carat assay. The yield, computed in relation to the current consumed, is about 35% In either of the above-mentioned examples, one obtains brilliant coatings which are ductile, irrespective of their thickness, free from cracks, which do not scale off and become detached when they are machined. When heated, they do not blister, even when their thickness exceeds 7 microns.

On the contrary, when operating in accordance with the examples, but with baths where there are no nitroguanidine and no nitro-urea, defective deposits are obtained. Those having a thickness exceeding 6 or 7 microns present, after quick heating at about 400 C., a quantity of large blisters; these can be emptied and one finds that the gas included therein is hydrogen.

At 10 to 12 microns thickness already'the deposits are fragile and if they must be machined one finds that they scale off or go away along the part touched by the tool.

If the thickness of the coatings is still greater, it often happens that cracks or scaling ofi occur while the object to be plated is still in the bath or shortly after its having been removed from the bath; these defects occur even with deposits the thickness of which is well under 20 microns.

What we claim is:

l. A process for the electrolytic deposition of a mem ber selected from the group consisting of gold and gold- 99% alloysv predominantly: of goldwhich Comprises passing. an electrolytic current through an aqueous solution containing an aikalinecyanide and'acyanideof said- 4;, dissolved state, a; small quantity of a nitrocompound selected from the group consisting of nitro-guanidine and nitro-urea.

5. A bath according to claim 4, containing 5 to 10 grams per litre of the said nitro compound.

6. A bath according to claim 4, wherein the said nitro compound is nitro-guanidine;

7. A bath according to claim'4, wherein the said nitro compound is nitro-urea.

References Cited in the file of this patent UNITED STATES PATENTS 1,903,860 Gockel' Apr; 18, 1933 

1. A PROCESS FOR THE ELECTROLYTIC DEPOSITION OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF GOLD AND GOLDCOPPER ALLOYS PREDOMINANTLY OF GOLD WHICH COMPRISES PASSING AN ELECTROLYTIC CURRENT THROUGH AN AQUEOUS SOLUTION CONTAINING AN ALKALINE CYANIDE AND A CYANIDE OF SAID SELECTED MEMBER, AND CONTAINING, IN SOLUTION IN THE BATH, A SMALL QUANTITY OF A MITRO COMPOUND SELECTED FROM THE GROUP CONSISTING OF NITRO-GUANIDINE AND NITRO-UREA. 